On vessels travelling from Europe to USA and wishing to receive satellite transmitted TV signals, the receiving antenna must be changed, due to the fact that the TV signals in Europe are linearly polarized and those at the USA are circularly polarized. Hitherto, the receiver (LNB) of the antenna—or at least part of it—has usually been physically replaced. A depolarizer is used in the USA to provide linearly polarized radiation for receipt. This is not required in Europe, but in Europe, however, the LNB must be rotated around its longitudinal axis (bore sight), if the vessel rolls about this axis, in order to be able to receive the linearly polarized radiation (the direction of polarization of which is dictated by the satellite and thus is constant during the rolling of the vessel).
In USA, rolling of the vessel is not a problem, as the radiation is circularly polarized, whereby the LNB is not required rotated.
In U.S. Pat. No. 4,806,945, a rotatable depolarizer positioned in the receiving head for providing a receiving head for use with both circularly and linearly polarized radiation. In this patent, the depolarizer is rotated by a motor.
It is noted that in the “linear polarization mode”, the depolarizer is positioned either parallel to or perpendicular to the polarization direction of the detector, so that the depolarizer is “invisible” to the radiation. In the “circular polarization mode”, the depolarizer is 45° to the polarization direction of the detector in order to convert the circularly polarized radiation to linearly polarized radiation before detection.
Naturally, the same technology may be used for transmitting signals to the satellite(s). In this respect, the depolarizer may be used for adapting the radiation output by a transmitter/antenna, such as a Block UpConverter. Then, angling the depolarizer 45° to linearly polarized radiation will convert this to circularly polarized radiation. Also, angling the polarizer 0° or 90° to the direction of polarization will render the depolarizer “invisible” to the radiation. Thus, the “direction of radiation” may be reversed from a transmitter to a receiver.